1. Field of the Invention
The present invention relates to an intake device of a two stroke engine.
2. Description of the Related Art
Known is a four stroke engine equipped with a turbocharger which comprises a turbine arranged in the exhaust passage of the engine, and a compressor arranged in the intake passage upstream of the throttle valve of the engine. In this engine, when the engine is operating under a light or a middle load, and thus the turbocharger is not operated, the amount of air fed into the engine cylinders is controlled by changing the position of the throttle valve from the idling position to the fully open position. In addition, in this engine, a bypass control valve is arranged in the bypass air passage bypassing the compressor of the turbocharger, and when the engine is operating under a heavy load and thus the turbocharger is operated, the charging pressure of air fed into the engine cylinders is controlled by changing the degree of opening of the bypass value while maintaining the throttle valve at the fully open position (see Japanese Unexamined Patent
Publication No. 61-83460). As mentioned above, in a four stroke engine, when the engine is operating under a heavy load, usually a turbocharger is operated to charge air fed into the engine cylinders.
In a two stroke engine equipped with an intake valve, however, the required pressure of air fed into the engine cylinders is different from that in a four stroke engine. Namely, in such a two stroke engine it is necessary to maintain the pressure in the intake part at a positive pressure at all times, to force fresh air into the engine cylinders, and to this end, a mechanically driven supercharger continuously driven by the engine is used. Such a mechanically driven supercharger is usually arranged in the intake passage downstream of the throttle valve, and therefore, during idling wherein the degree of opening of the throttle valve is small, the fresh air of a large vacuum downstream of the throttle valve must be pressurized to a positive pressure by the mechanically driven supercharger. The amount of compression applied by the mechanically driven supercharger is proportional to the difference between the pressure on the suction side and the pressure on the discharge side of the mechanically driven supercharger, and therefore, the bigger this difference in pressure, the larger the amount of compression needed by the mechanically driven supercharger. Consequently, where the mechanically driven supercharger is arranged in the intake passage downstream of the throttle valve, if the engine is operating under a light load, as when idling, since the difference between the pressure on the suction side and the pressure on the discharge side of the mechanically driven supercharger becomes great, the amount of compression needed by the mechanically driven supercharger is increased. As a result, the loss of engine power for driving the mechanically driven supercharger is increased, and thus a problem occurs in that the fuel consumption will be increased. Further, when the amount of compression by the mechanically driven supercharger is increased, the temperature of the air discharged from the mechanically driven supercharger becomes high, and thus another problem occurs in that knocking or self-ignition will occur. Consequently, to avoid the above-mentioned problems, it is preferable to lower the level of vacuum generated on the suction side of the mechanically driven supercharger. But, if the level of vacuum produced on the suction side of the mechanically driven supercharger is excessively lowered, another problem occurs in that the engine speed can not be stabilized.
Namely, when a vacuum produced on the suction side of the mechanically driven supercharger, i.e., a vacuum produced in the intake passage downstream of the throttle valve, is great the velocity of air passing through the throttle valve becomes close to the velocity of sound, and as a result, the engine speed has little influence on the amount of air passing through the throttle valve, and thus this amount of air is maintained at an approximately constant value. Consequently, as illustrated by the curved lines A and B in FIG. 14, if the engine speed NE is increased beyond NE.sub.0, since the amount of air fed into the engine cylinders per one revolution of the engine is decreased, the output torque of the engine is reduced, and thus the engine speed NE is reduced. Conversely, if the engine speed NE is decreased below NE.sub.0, since the amount of air fed into the engine cylinders per one revolution of the engine is increased, the output torque of the engine is increased, and thus the engine speed NE is increased. Consequently, the engine speed NE is maintained at NE.sub.0. In FIG. 14, the curved line A illustrates the case wherein a vacuum produced in the intake passage downstream of the throttle valve is great, compared with the case illustrated by the curved line B, and thus it will be understood from FIG. 14 that the engine speed NE becomes more stable as a vacuum produced in the intake passage downstream of the throttle valve becomes greater.
Conversely, when the level of vacuum produced in the intake passage downstream of the throttle valve is small, even if the engine speed NE is increased beyond NE.sub.0, and a vacuum produced in the intake passage downstream of the throttle valve becomes greater, the amount of air fed into the engine cylinders per one revolution of the engine is not greatly decreased. Consequently, as illustrated by the broken line C in FIG. 14, the output torque of the engine is not greatly decreased. Further, when the engine speed NE is decreased below NE.sub.0, the output torque of the engine is not greatly increased. Therefore, when the engine speed NE deviates from NE.sub.0, the engine speed NE can not be easily returned to NE.sub.0, and thus the engine speed NE is not stabilized.
Consequently, to reduce the loss of engine power for driving the mechanically driven supercharger, prevent knocking or self-ignition, and stabilize the engine speed, it is necessary to maintain the level of vacuum produced in the intake passage downstream of the throttle valve at a fixed level which is not too high and not too low. The above-mentioned Japanese Unexamined Patent Publication No. 61-83460, however, does not teach the concept of controlling the level of a vacuum produced in the intake passage downstream of the throttle valve.